As part of a standardized research sequence in the NIMH Intramural clinical program, patients with schizophrenia are withdrawn from neuroleptic medication and, after clinical relapse, treated with a fixed dose of haloperidol. This in turn allows a variety of studies regarding the pharmacokinetics of haloperidol and the effects of haloperidol and other neuroleptics on central nervous system catecholamines. Another pharmacological issue being examined in these patients is drug-drug interactions (specifically involving haloperidol and several other drugs, including nicotine, caffeine, retinoic acids, and ascorbic acid). In addition, basic science investigations regarding these drugs are being conducted.